Rubber-modified styrene resin, a representative impact resistant polystyrene resin, is characterized by excellent plasticity, strength, and electrical properties. Owing to these characteristics, this resin has been widely applied to various industrial fields including office automation devices such as word-processors, personal computers, printers, copiers, etc; home electronic appliances such as TVs, VCRs, audio systems, etc; electronic parts; and auto parts.
Even with these excellent plasticity and mechanical properties, the flammable rubber-modified styrene resin has a safety problem. Therefore, efforts have been continuously made to develop a flame retardant rubber-modified styrene resin. The flame resistance has been controlled by UL and the method in which halogen flame-retardants are mixed with styrene resin together with a flame retardant auxiliary has been the most commonly accepted. The halogen flame-retardant is selected from a group consisting of polybromodiphenylether, tetrabromobisphenol A, Br-substituted epoxy compound and chlorinated polyethylene. The flame retardant auxiliary is an antimony compound such as antimony trioxide and antimony pentoxide.
The method of generating flame-resistance by mixing a halogen flame retardant with the antimony flame retardant auxiliary has been successful at endowing excellent flame-resistance without damaging the mechanical properties, but there are chances of damaging the mold from hydrogen halide during the processing, and producing dioxin, a strong carcinogen, from the waste incineration, causing environmental and biological problems. Recently, actions have been taken to restrict such halogen flame retardants and thus there is a need to develop a flame retardant resin excluding halogen elements.
To endow flame resistance to a rubber-modified styrene resin that does not include halogen elements, a rubber-modified styrene resin has been blended with polyphenylene ether, which is expected to improve flame resistance and heat resistance.
U.S. Pat. No. 3,639,506 and No. 3,883,613 describe that it is effective to add an aromatic phosphoric ester such as triphenylphosphate and trimesitylphosphate as a flame retardant to the blend of rubber-modified styrene resin and polyphenylene ether resin. U.S. Pat. No. 6,124,385 describes that flame resistance is improved by adding a free radical generator and triphenylphosphate or triphenylphosphineoxide alone or together to the mixture of rubber-modified styrene resin and polyphenylene ether resin.
During the processing of the polyphenylene ether and rubber-modified styrene blend into a flame-retardant resin, the processing temperature is high because of the heat-resistant polyphenylene ether and the screw has to be separately designed to handle the huge shear force. At this time, if the processing temperature is 250° C. or higher, the low molecular aromatic phosphoric ester will evaporate, suggesting that the final content of the compound in the final resin is much less than at the early stage, so flame resistance and the physical properties will be altered. An increase of polyphenylene ether content is a disadvantage for preparing a flame retardant resin for dripping because the over-dose of polyphenylene ether inhibits the dripping of the flame retardant resin.
In the course of studies on the development of a flame retardant rubber-modified styrene resin with excellent flame resistance for dripping and without polyphenylene ether to overcome the above problems, the present inventors completed this invention by confirming that the flame resistance for dripping of a resin is greatly improved when a phosphoric ester compound is used as a flame retardant and an aliphatic amide compound is used as a dripping enhancer without damaging heat resistance and fluidity.